In general, spark plugs have a center electrode and a ground electrode in a front end portion thereof and a terminal nut, for receiving supply of electric power, in a rear end portion thereof. The terminal nut is held in an axial hole of an insulator and protrudes from a rear end of the insulator. The insulator is accommodated and held in a metallic shell. A flat portion is formed at the rear end of the insulator, and a contact surface of a stepped portion of the terminal nut is in contact with the flat portion of the insulator.
The terminal nut is fixed to the inside of the axial hole of the insulator by a heat sealing process. In the heat sealing process, in a state in which a front end portion of the insulator is oriented downward, first, a center electrode is inserted into a front end portion of the axial hole of the insulator. Resistor powder and electroconductive sealing powder are then put into the axial hole. Subsequently, the terminal nut is inserted into the axial hole in such a way that the terminal nut protrudes from the rear end of the insulator. Next, while pressing the terminal nut downward, the resistor powder and the electroconductive sealing powder are heated to be softened and then cooled to be solidified, and thereby the center electrode and the terminal nut are sealed and fixed to each other in the axial hole of the insulator. The insulator, in which the center electrode and the terminal nut have been fixed to each other in this way, is fixed to the metallic shell by a crimping process. In the crimping process, a crimping portion at the rear end of the metallic shell is crimped, and a buckling portion of the metallic shell is buckled. As a result, the metallic shell and the insulator engage each other securely. In the crimping process, in order to hold the insulator at a correct position, crimping is performed while pressing the terminal nut at the rear end by using a pressing jig.
Regarding spark plugs, various technologies have been developed in order to suppress flashover (surface creepage that occurs between the terminal nut and the metallic shell along the surface of the insulator) and to prevent breakage of the insulator (See Japanese Unexamined Patent Application Publication No. 2003-45609; Japanese Unexamined Patent Application Publication No. 2013-16295; and Japanese Unexamined Patent Application Publication No. 2013-131375).
In recent years, spark plugs have been reduced in size and diameter for the purpose of increasing flexibility in the design of internal combustion engines. As the diameter of a spark plug is reduced, the thickness of the insulator is reduced, and therefore a problem arises in that the strength of the insulator is reduced. Moreover, various parts of the spark plug are required to have a higher dimensional accuracy and a higher assembly accuracy. Regarding the assembly accuracy of the spark plug, the eccentricity between the terminal nut and the insulator after the aforementioned heat sealing process is particularly important. That is, when the eccentricity between the terminal nut and the insulator increases, it is likely that required assembly accuracy cannot be satisfied in the aforementioned crimping process. To be more specific, if the eccentricity between the terminal nut and the insulator is large, the terminal nut (and the insulator) cannot be held at a correct position in the crimping process, and the insulator may be fixed to the metallic shell in a state in which the insulator is considerably displaced.
There is also a problem in that flashover becomes more likely to occur as the eccentricity between the terminal nut and the insulator increases. That is, a flat portion, which comes into contact with a contact surface of a stepped portion of the terminal nut, is formed at an insulator head (the rear end of the insulator). The flat portion of the insulator head, which has an outside diameter larger than that of the terminal nut, has a function of suppressing flashover. However, if the eccentricity between the terminal nut and the insulator is large, the assembled shape is equivalent to a shape in which the outside diameter of the flat portion of the insulator head is effectively small, so that a problem arises in that flashover becomes more likely to occur.
The present invention, which has been devised to address the aforementioned problem, can be implemented as follows.